Several integrated circuits are required to be driven by different voltage references in order to immunize the circuits from the noise due to fluctuations of the power voltage references (due to the switching of the downstream power transistors) or for particular application in which devices have a floating voltage reference (e.g. ICL telemetry).
For example, U.S. Pat. No. 5,276,405 (incorporated by reference) discloses an integrated amplifier which includes an input stage with two BJT transistors Q1, Q2 each biased by a constant current IP3, IP4 while the collectors of the transistors Q1, Q2 are kept at fixed voltage references. A differential input signal IN(+)-IN(−) applied between the emitters of the transistors Q1, Q2 is transferred to the terminals of a first resistor R1 which is supplied with a bias current IR1 from a circuit formed by transistors Q5, Q6, Q7 which mirrors the bias current to a second resistor R6, from the terminals of which the output signal VOUT is taken. While the input signal IN(+)-IN(−) is differential, the output signal VOUT will be referred to a ground reference G.
The primary object of the invention disclosed in the above mentioned patent is to avoid the use of a second supply so that the amplifier can be supplied with a single supply voltage, while retaining the characteristics of high input impedance and dynamic range with equally high CMRR and PSRR values. However, the integrated amplifier could be also adopted to perform a conversion between voltage signals referred to different voltage references, in this case, from a differential input signal to a ground referred output signal.
Unfortunately, said known integrated circuit, where the input signal is provided by the emitters of the two transistors included in the input stage, are affected by the problem of an excessive current consumption at the input terminals. Moreover, a limited input voltage range is permitted before the breakage of the transistors of the input stages.